This invention relates to fluid control valves, and more particularly to gate valves with backseats to establish a pressure seal between the valve stem and valve bonnet.
Selective backseat gate valve designs typically backseat the valve stem against a metallic or non-metallic seal surface in the valve bonnet. This backseat action isolates the stem packing stuffing box from the valve bore pressure and thereby permits replacement of the stem packing while the valve remains pressurized. For improved safety, some valves are designed to mechanically lock the backseat in its engaged position prior to initiation of the stem backing replacement operation. But, this mechanical lock is typically only achievable if the valve is in the fully closed or fully open position, depending upon the specific valve design. If these designs are backseated in any intermediate position, the backseat cannot be mechanically locked in place and may therefore become "deenergized" if the stem is accidentally jarred during the packing replacement operation, whereby leakage will then occur.